1. Field of the Invention
This invention relates to a coiled spring, and particularly to an improvement in a spring wire intended for use in making a coiled spring in which a wire is generally oval in section.
2. Description of the Prior Art
Ordinarily, a conventional coiled spring is made from a wire which is circular in section. However, there was a problem in that as is known, when an axial load acts on the coiled spring of circular section, a stress generated in the peripheral portion is large internally of the coil due to the curved coil wire and direct shearing force, and therefore, not only the energy efficiency is poor but cracks which lead to snapping are likely to occur.
The maximum stress .tau..sub.max is obtained by Wahl formula: ##EQU1## where C is the spring exponent, and C is given by EQU C=D/d
D is the diameter of coil and d is the diameter of wire.
Japanese Patent Publication No. 3261/52 and U.S. Pat. No. 2,998,242 have been known which improve a disadvantage in that the maximum stress increases. In the former, the wire is oval in section, and in the latter, the shape is a combination of a semicircle and a semiellipse. The relation between the long diameter W and the short diameter t of the wire is determined by ##EQU2##
On the other hand, with the recent tendency of reducing the weight of automobiles, it has been required, in valve springs, torsion springs and the like of the engine, to design light-weight springs. This means that the length of a spring when the wire turns are in close (actual) contact, i.e. when the spring is compressed, called the solid height, is minimized and the weight absorbing a given amount of energy is minimized, that is, the energy efficiency is enhanced.
The length H.sub.s, or solid height, of the spring is generally calculated by EQU H.sub.s =(N-0.5)t (3)
where N is the total number of turns of the coil, and t is the longitudinal dimension of wire.
That is, to reduce the said height, the total number of turns N is reduced and the longitudinal dimension of the wire is made small. To enhance the efficiency of energy, the stress in the peripheral portion is made uniform, and the maximum stress .tau..sub.max is lowered.
In the shape of a wire comprising a semicircle and a semiellipse as described above, the said height can be reduced by flattening a section of the wire. This meets the aforementioned requirement in respect of the fact that the maximum stress is lowered, to which attention is recently invited suddenly.
In the conventional material-dynamics solution, stress analysis of a suitable sectional shape is extremely difficult, and the shape of a wire is also considered to be relatively simple. However, lately, there is established an elastic-dynamics solution (Fourier development boundary value average method) in which a boundary in the outer peripheral portion in section is divided into a number of wire elements, Fourier development is made along each of the wire elements, which are expanded over the whole area of boundary, thus making it possible to perform stress analysis of a suitable shape.
As a consequence, it has become clear that the above-described known semicircular and semielliptical sectional shapes are not always sufficient in respect of evenness of stress and reduction in maximum stress